The weighting is 2-to-3 for every TC, so when a rune cascades down the Rune TC list and stops on a set of two runes, it only has a 40% chance to choose the less common one this is why Ber is a lower rune than Jah, but is actually slightly rarer. If a monster can drop one, it can drop the other as well. Rune TC's each contain two runes, besides Zod's TC. Also, these are the odds of dropping each rune from rune drops which could be as high as Zod (mlvl 81+):

The following ilvls are the minimum for dropping High Runes. This makes it so that you can find Zod Runes as early as act 1 hell, for example. Other monsters will have their TC upgraded to include higher rune TC's, depending on their mlvl.

You may have heard about certain runes being delegated to certain acts of the game, but this is only true for chests, without exception (including superchests). Many people often ask what level HR's can drop at, since they are not found in any item TC with a simple label such as armo63 or weap87, but rather Rune TC's. This is one of the upsides of multiplayer - certain unused HR's have more value than just being cubed up (notably Lo -> Sur, Ber -> Jah). Note that Runes dropping in multiplayer have about 25% more value per-drop on average when using these listed values. Ber is worth far more than half a Jah, so it would never be cubed in Multiplayer.) I made the following table to show the relative values of all runes above Mal when considered in Multiplayer, in terms of Sur Runes (since everyone is after Ber, so Sur is always in demand for cubing into it) In Multiplayer, certain runes have additional value since they are more in-demand than just needing to be cubed.

But first, some important info about High Runes and how we need to value them in this post: This guide will compare the seven most efficient methods for finding High Runes in the game.

Diablo 2 farm runes